In order that the instant invention may be clearly understood, it is useful to review the prior art relating to photochemical isotope separation. U.S. Pat. No. 2,713,025 and British Pat. No. 1,237,474 are good examples of processes for the photochemical separation of the isotopes of mercury. The first requirement for photochemical isotope separation is that one finds conditions such that atoms or molecules of one isotope of a given element absorb light more strongly than do atoms or molecules of another isotope of said element. Mercury is a volatile metal and readily forms a vapor of atoms. Said atoms absorb ultraviolet light at 2537 A. The absorption line of Hg.sup.202 is displaced by about 0.01 A with respect to the absorption line of Hg.sup.200. Since the absorption lines are extremely narrow, one may by use of a light in a critically narrow wavelength region excite either Hg.sup.200 or Hg.sup.202.
The second requirement for a photochemical isotope separation is that those atoms or molecules which are excited by light undergo some process which the atoms or molecules which have not been excited do not undergo, or at least do not undergo as rapidly. A quantum of 2537 A ultraviolet light imparts an excitation of 112.7 Kcal/mole to the mercury atom which absorbs it. The number of mercury atoms which at room temperature are thermally excited to this energy is vanishingly small, hence the atoms excited by light are not diluted by atoms excited by thermal means. Atoms of this high excitation readily undergo reactions with H.sub.2 O (as taught in the U.S. patent) or with O.sub.2, HCl or butadiene (as taught in the British patent), said reactions not occurring at room temperature with unexcited mercury.
Uranium, however, is a highly refractory metal, boiling only at extremely high temperatures. Thus, use of the above-described process with uranium atoms instead of mercury involves obvious difficulties. The most volatile form of uranium is UF.sub.6. U.sup.235 F.sub.6 and U.sup.238 F.sub.6 both absorb ultraviolet light and do so to exactly the same extent at all wavelengths in the UV; hence, UV excitation of UF.sub.6 does not satisfy the first requirement of photochemical isotope separation. However, UF.sub.6 will also absorb infrared light in the region around 626 cm.sup.-1 (the V.sub.3 band) and 189 cm.sup.-1 (the V.sub.4 band). Both the V.sub.3 and V.sub.4 bands of U.sup.235 F.sub.6 are shifted slightly toward higher energy with respect to the V.sub.3 and V.sub.4 bands of U.sup.238 F.sub.6 respectively, but the size of these shifts is small compared to the width of the bands; in other words, the infrared absorption spectra of U.sup.238 F.sub.6 and U.sup.235 F.sub.6 do not exactly coincide, but they overlap at all wavelengths so that if one isotope absorbs light, so, to a substantial degree, will the other. Hence, the infrared excitation of UF.sub.6 by absorption of a single IR photon is a process of limited isotopic selectivity.
The second requirement for isotope separation is also a matter of some difficulty for UF.sub.6. UF.sub.6 molecules which are excited by IR light are no different from molecules which have received the same energy by thermal excitation. Whatever process the photo-excited molecules will undergo, those molecules which are thermally excited to the same energy will also undergo. This dilution of the photo-excited molecules with thermally excited molecules will further decrease the isotopic separation factor.
If the irradiation conditions are such that molecules may acquire the energy of several IR photons then the molecules may be excited to energy levels but little populated by thermal means and dilution of photo-excited molecules by thermally excited molecules is minimized. This advantage of multiple photon absorption is recognized by the prior art as discussed below. It is however, to be recognized that there is more than one way by which multiple photon absorption may be achieved and the isotopic selectivity of the process depends on which way is used.
To achieve multiple photon absorption two difficulties must be overcome. First it is well known that excited molecules may become deexcited by any of several rapid means. Thus if the excitation process is to convert singly excited molecules to doubly excited molecules and thence to triply excited molecules etc., then the excitation process must be rapid compared to the deexcitation processes. This implies that the molecules must be irradiated with infrared light at a high power density.
The second difficulty is more complex. In order to provide light of the required high power density it is necessary to use a laser. Lasers are also noted for their ability to produce light at a single exactly defined wavelength. However in molecular spectroscopy there is an effect known an anharmonicity. Because of this effect a molecule which has absorbed an Ir photon at some wavelength .lambda. to become singly excited, has an absorption spectra which is, in general, shifted toward lower energies. Thus to repeat the absorption step and make the molecule doubly excited a second photon of wavelength .lambda.' is needed while to triply excite a third photon of wavelength .lambda." is required.
It is to be understood that all methods for achieving multiple photon absorption have high power requirements, but the quantitive definition of that requirement will vary from method to method. The major distinction in said methods is the means by which anharmonicity is overcome.
The simplest way to overcome the anharmonicity problem is to use a laser which emits not a single exact wavelength but a finite range of wavelengths. The range of wavelengths a laser emits is called the bandwidth. If, in the above example, the bandwidth is sufficient to include .lambda.,.lambda." and .lambda."' then clearly a three photon absorption process is possible. In considering the effective bandwidth for multiple photon absorption processes, the well-known effect of power broadening must also be taken into account. In two references known in the prior art this type of multiple photon absorption appears to have been achieved. See Lyman et al, Applied Physics Letters 27, 87, 1975 and Ambartzuminan et al. in Soviet Physics JETP 21, 375, 1975. Both these reference report experiments in which SF.sub.6 is dissociated in an isotopically selective manner by high power radiation from a CO.sub.2 laser. The conditions used were such that substantial power broadening would occur and since the energy required to dissociate the SF.sub.6 molecule is that of many IR photons it is apparent that multiple photon absorption has occurred. It is not completely clear whether power broadening was the sole cause of the multiple photon absorption or whether other and unknown processes may also have contributed.
It will readily be appreciated that for purposes of isotope separation, UF.sub.6 and SF.sub.6 are entirely nonequivalent substances. According to Klimov and Lobikov, Optics and Spectroscopy, 30, 25 (1971) S.sup.32 F.sub.6 has its .nu..sub.3 absorption band at 947 cm.sup.-1 while S.sup.34 F.sub.6 has its .nu..sub.3 absorption band at 930 cm.sup.-1. Although both bands have a finite width, they do not significantly overlap because of the large 17 cm.sup.-1 separation. The corresponding .nu..sub.3 absorption band in UF.sub.6 occurs at 626 cm.sup.-1 and according to McDowell et al (Journal of Chemical Physics, 61, 3571 (1974), the .nu..sub.3 band of U.sup.235 F.sub.6 is shifted by 0.65 cm.sup.-1 with respect to the .nu..sub.3 band of U.sup.238 F.sub.6, however each of the bands has a width at half height of 14 cm.sup.-1. Thus at any wavelength at which one isotopic uranium molecule absorbs light so will the other to a comparable, although not exactly equal, degree. Thus it is not obvious from the experiments of Ambartzumian et al, that any useful separation of uranium isotopes is possible by photochemical means. Further, this reference neither teaches, shows, nor suggests any means for obtaining a useful photochemical isotope separation in situations where the absorption bands strongly overlap as is the case for UF.sub.6.
The process of the instant invention, on the other hand, is especially suitable for the isotopic separation of elements having an atomic number of 70 or greater, i.e., elements wherein the isotope shift is very small and thus the absorption bands overlap.
U.S. Ser. 408,669 and the Continuations-in-part thereof (see above) teach a means by which anharmonicity may be overcome to provide an isotope separation process wherein multiple photon absorption yields an increased isotopic selectivity over the prior art processes based on single photon absorption. In these processes, a second gas is utilized to promote rotational relaxation between the absorption of IR photons. The intervening rotational relocation allows molecules in some rotational state J to absorb a photon at wavelength .lambda. and become singly excited, then change their rotational state to J' and absorb a second photon also at .lambda. and become doubly excited, change their rotational state to J" absorb a third photon to become triply excited, etc. It can be shown that the isotopic selectivity with which single photons are absorbed is related to the rotational distribution, thus since the rotational distribution is continually reestablished each step of photon absorbtion may be isotopically selective and the selectivity of the multiple photon absorbtion process may be the result of compounding the selectivity of the individual steps.
The instant invention also teaches a process in which anharmonicity is overcome. Thus the instant process also obtains the increased isotopic selectivity of multiple photon absorption as compared to the isotopic selectivity of the prior art single photon absorption processes in a manner entirely different from that taught in U.S. Ser. No. 408,669 and the Continuations-in-part thereof.
The differences between the process taught in U.S. Ser. No. 408,669 and the Continuations-in-part thereof include: U.S. Ser. No. 408,669 requires the presence of a second gas and the instant invention has no such requirement. In U.S. Ser. No. 408,669 there is no requirement as to the bandwidth of the laser, but for the instant invention a minimum bandwidth of 0.1 cm.sup.-1 is required. In U.S. Ser. No. 408,669 it is preferred to irradiate the molecules with radiation which falls within the R branch of the molecular absorption band, while the instant invention requires the use of radiation which falls within a P branch of the molecular absorption band.